This invention relates to an improved cationic exchange membrane, a process for making that membrane, and apparatus which is used in the process. More particularly, the membrane is a fluorinated cation exchange membrane, one important use of which is to separate the anode and cathode compartments of a chloralkali cell.
Fluorinated polymers containing pendant side chains containing sulfonyl groups are now well-known, and their use as ion exchange membranes is also known. It is desirable to have an ion exchange membrane which is supported, i.e., which contains a material which imparts physical strength to the fluorinated polymer, so that the physical strength of the complete membrane construction is greater than that of a film of the fluorinated polymer. Heretofore, methods for supporting such membranes have not been adequate, for if films of desirable thickness were employed complete encapsulation of the support material was not effected, and if complete encapsulation of the support material were to be assured excessively thick films of fluorinated polymer were required. Such excessive film thickness not only increases the cost of the membrane, but it also reduces the usefulness of the membrane for ion exchange purposes because the increased thickness leads to higher operating voltage and higher power consumption. If the support material is not completely encapsulated, the membrane will leak or will in use ultimately rupture at the non-encapsulated points and will then leak, and thus its usefulness is impaired.
A method which has been proposed to overcome the above problems and deficiencies is that of U.S. Pat. No. 3,770,567 wherein a film of fluorinated polymer which contains pendant side chains containing --SO.sub.2 L groups, where L is F or Cl, is treated on one surface with an alkali metal hydroxide, an alkaline earth metal hydroxide or ammonium hydroxide, to form a hydrolyzed surface layer wherein the functional groups are in the --(SO.sub.3)jM form, where M is alkali metal, alkaline earth metal or ammonium, and j is the valence of M, followed by contacting the --SO.sub.2 L surface of the film with a support material, and applying a differential pressure to the contacted support material and the film, the pressure on the opposite surface of the support material from that which is contacting the fluorinated polymer being at least 5 inches (127 mm) of mercury less than the pressure on the surface of the fluorinated polymer film opposite to that contacting the support material, for a sufficient period of time to cause the support material which is in contact with said film to become completely encapsulated within the film of fluorinated polymer while heating the film and support material at a temperature of from 240.degree.-320.degree. C. The resulting laminate is then subjected to a second hydrolysis treatment with alkali metal hydroxide, alkaline earth metal hydroxide or ammonium hydroxide after which it is ready for use for ion exchange purposes. This method has the disadvantage of adding an additional processing step in the formation of the supported structure, and many additional hours of processing time are required to effect the surface hydrolysis step in the hydroxide treating bath used. Additionally this method cannot be used for fluorinated polymers which contain carboxylic functional groups because the hydrolysis step would lead to carboxylic acid groups or salts thereof, which easily decarboxylate at the temperatures employed in forming the supported construction.
It is a principal object of the invention to provide novel web supported membranes of exceptional uniformity. In the novel web reinforced membranes the sulfonyl and carboxyl groups can be either in melt fabricable form, or, after hydrolysis or other suitable chemical reaction, can be in ion exchange form.
It is another object of this invention to provide a process for forming a supported structure of fluorinated polymers which contain pendant side chains containing either sulfonyl groups or carboxyl groups or both, which method leads to a completely encapsulated supported structure, and which eliminates the necessity for a surface hydrolysis step.
It is a further object to provide apparatus adapted for carrying out the process of the invention specified immediately above.
Other objects will be apparent from the continuing description.